In a system that electromechanically scans a light beam over a document, such as an optical scanning, reading and recording system used, for example, in a facsimile reader or printer, accurate control of the scanning device is essential.
Many low cost, high speed systems use a galvanometer scanner. The performance of a high speed galvanometer scanner is very sensitive to temperature variations, performance history, load, inertia and other factors. As a result, it is often necessary to incorporate the galvanometer in a closed loop feedback system.
Several methods have been attempted to provide a relatively accurate feedback signal for the galvanometer. One method involves sensing galvanometer position by measuring variable capacitance formed between a moving plate fixed to a galvanometer shaft and a fixed plate attached to the galvanometer frame. This method adds inertia to the galvanometer thus limiting the galvanometer's ultimate scanning frequency. An example of a galvanometer utilizing a capacitance in the stator structure of the scanner is discussed in a paper entitled Applications of Galvanometers to Laser Scanning by Pierre J. Brosens and Edward P. Grenda, presented at the Eighteenth Annual Technical Meeting, Society of Photo-Optical Instrumentation Engineers, Aug. 19-23, 1974, at San Diego, Calif.
Another method for sensing the shaft position of a galvanometer includes the use of the shaft angular velocity to recover the back EMF from the galvanometer winding by subtracting the various other electrical components comprising the galvanometer's voltage. A third method utilizes separate windings to recover the back EMF which is proportional to the galvanometer velocity. Each of the above methods have been demonstrated to contain excessive drift with temperature and excessive instability as a function of galvanometer environmental history.